1. Field of Invention
The present invention relates generally to steam cookers, and more particularly to a steam cooker in which the flow of steam into an oven chamber in which food is received is modulated as a function of the quantity and temperature of the food in the chamber to effect efficient cooking thereof.
2. Status of Prior Art
Steam is water vapor, this being the gaseous phase of water which occurs when water is boiled. The freezing and boiling points of water at atmospheric pressure determine respectively the zero degree and hundred degree points on the centigrade scale. The cloudy mist seen when water boils is not truly steam which is a colorless and invisible gas, but consists of fine water droplets. Hence true steam is sometimes referred to as dry steam, and when dry steam is beginning to condense back to water droplets, it is then referred to as wet steam. In the present invention the term steam is applicable to both wet and dry steam.
The use of a "pressure cooker" to cook food is well-known, this being an air tight pot or oven chamber that uses steam under pressure at a high temperature to cook food far more quickly than food can be cooked by steam in an unsealed oven at close to atmospheric pressure. One objection to a pressure cooker is that it is costly to make, for it must satisfy pressure vessel safety regulations.
But a more serious drawback of a pressure cooker is that in cooking frozen vegetables or other frozen products which must be thawed, the transfer of thermal energy with pressurized steam at high temperatures is so rapid that before the core of the frozen food is adequately cooked, the exterior regions of the food begin to overcook. Hence pressure cookers often fail to cook food properly.
Steamers which are not pressure cookers, though slower to cook than pressure cookers are far less dangerous to use. A pressure cooker becomes hazardous when the internal steam pressure reaches a level well above the pressure rating of the vessel, at which point the pressure cooker might explode. Moreover, conventional pressureless steamers make it possible for a chef to operate the steamer as he does a conventional oven so that its door can be opened and closed without being concerned about the release of internal steam pressure.
U.S. Pat. No. 5,368,008 discloses an atmospheric convection type steamer having at least one oven chamber and a discrete atmospheric steam generator or boiler placed below the oven from which steam is piped up to the oven. The steam delivery efficiency of this known arrangement is not high, for there is a loss of thermal energy when conveying steam through pipes.
In a conventional pressureless steamer in which steam is supplied to an oven by a boiler and the oven is provided with an access door, when the oven is opened to remove the cooked food, steam is then released to the atmosphere. This rush of steam may cause severe burns on the hands and face of the operator unless he exercises care in positioning himself away from the oven when opening its door. But when the operator is a busy chef, he may on occasion fail to stand back and away from the oven when opening its door.
Another problem encountered with conventional pressureless steam cookers is excessive water consumption. In such cookers, water boiled in a steam generator is piped into an oven in which the food is received, and steam is drained from the oven through a drain pipe leading to a waste line. Many local codes which regulate the operation of steam cookers and other water heating devices, interdict feeding water down a drain whose temperature is above 140.degree. F. To comply with these regulations, the present practice is to spray cooling water into the steam exit to reduce the temperature of the condensate fed into the drain line to an acceptable lower level. This gives rise to water consumption in commercial steam cookers that may run as high as 58 gallons per hour, regardless of the quantity of food being cooked.
Moreover, the thermal efficiency of a conventional pressureless steam cooker is not high, for regardless of the quantity of food being steamed in the oven the flow of steam into the oven from a steam generator is continuous. This is wasteful of thermal energy, for in the initial phase of cooking a food product that is at room temperature or is in a frozen state, the volume of steam required at the outset of cooking must be high. But when the food produced is thereafter in a heated condition, less steam is then required to complete the cooking operation. Thus conventional pressureless steam cookers do not take into account the varying demand for steam in the course of a cooking operation.
Another factor which impairs the efficiency of a steam cooker is the build-up of lime on the electric heater coils or the tubes of the steam generator because of the mineral content of the water being boiled. The resultant lime coating acts as a thermal insulating layer, thereby causing high-watt density heaters which are immersed in water to overheat and self-destruct. In some instances, the water is boiled by a gas-fired water-tubes boiler. The present practice is to remove the lime coating on the heater coils or the boiler tubes by means of a chemical solvent. But before the steam generator can be put back into service, the solvent must be completely flushed away, otherwise the solvent will contaminate the steam. This need to completely wash away the solvent presents practical difficulties.